Search for a hypothetical gauge boson and dark photons in charmonium transitions

We report a direct search for a new gauge boson, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>X</a:mi> </a:math> , with a mass of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mn>17</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>MeV</c:mi> <c:mo>/</c:mo> <c:msup> <c:mi>c</c:mi> <c:mn>2</c:mn> </c:msup> </c:math> , which could explain the anomalous excess of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msup> <e:mi>e</e:mi> <e:mo>+</e:mo> </e:msup> <e:msup> <e:mi>e</e:mi> <e:mo>−</e:mo> </e:msup> </e:math> pairs observed in the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:mmultiscripts> <g:mrow> <g:mi>Be</g:mi> </g:mrow> <g:mprescripts/> <g:none/> <g:mrow> <g:mn>8</g:mn> </g:mrow> </g:mmultiscripts> </g:mrow> </g:math> nuclear transitions. The search is conducted in the charmonium decays <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:msub> <i:mi>χ</i:mi> <i:mrow> <i:mi>c</i:mi> <i:mi>J</i:mi> </i:mrow> </i:msub> <i:mo stretchy="false">→</i:mo> <i:mi>X</i:mi> <i:mi>J</i:mi> <i:mo>/</i:mo> <i:mi>ψ</i:mi> <i:mo stretchy="false">(</i:mo> <i:mi>J</i:mi> <i:mo>=</i:mo> <i:mn>0</i:mn> <i:mo>,</i:mo> <i:mn>1</i:mn> <i:mo>,</i:mo> <i:mn>2</i:mn> <i:mo stretchy="false">)</i:mo> </i:math> via the radiative transition <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mi>ψ</n:mi> <n:mo stretchy="false">(</n:mo> <n:mn>3686</n:mn> <n:mo stretchy="false">)</n:mo> <n:mo stretchy="false">→</n:mo> <n:mi>γ</n:mi> <n:msub> <n:mi>χ</n:mi> <n:mrow> <n:mi>c</n:mi> <n:mi>J</n:mi> </n:mrow> </n:msub> </n:math> using <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mrow> <s:mo stretchy="false">(</s:mo> <s:mn>2712.4</s:mn> <s:mo>±</s:mo> <s:mn>14.3</s:mn> <s:mo stretchy="false">)</s:mo> </s:mrow> <s:mo>×</s:mo> <s:msup> <s:mn>10</s:mn> <s:mn>6</s:mn> </s:msup> <s:mtext> </s:mtext> <s:mtext> </s:mtext> <s:mi>ψ</s:mi> <s:mo stretchy="false">(</s:mo> <s:mn>3686</s:mn> <s:mo stretchy="false">)</s:mo> </s:math> events collected with the BESIII detector at the BEPCII collider. No significant signal is observed, and the updated upper limit on the coupling strength of charm quark and the new gauge boson, <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:msub> <y:mi>ε</y:mi> <y:mi>c</y:mi> </y:msub> </y:math> , is set to be <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mo stretchy="false">|</ab:mo> <ab:msub> <ab:mi>ε</ab:mi> <ab:mi>c</ab:mi> </ab:msub> <ab:mo stretchy="false">|</ab:mo> <ab:mo><</ab:mo> <ab:mn>1.2</ab:mn> <ab:mo>×</ab:mo> <ab:msup> <ab:mn>10</ab:mn> <ab:mrow> <ab:mo>−</ab:mo> <ab:mn>2</ab:mn> </ab:mrow> </ab:msup> </ab:math> at 90% confidence level. We also report new constraints on the mixing strength <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"> <eb:mi>ε</eb:mi> </eb:math> between the Standard Model photon and dark photon <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"> <gb:msup> <gb:mi>γ</gb:mi> <gb:mo>′</gb:mo> </gb:msup> </gb:math> in the mass range from 5 to <ib:math xmlns:ib="http://www.w3.org/1998/Math/MathML" display="inline"> <ib:mn>300</ib:mn> <ib:mtext> </ib:mtext> <ib:mtext> </ib:mtext> <ib:mi>MeV</ib:mi> <ib:mo>/</ib:mo> <ib:msup> <ib:mi>c</ib:mi> <ib:mn>2</ib:mn> </ib:msup> </ib:math> . The upper limits at 90% confidence level vary within <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"> <kb:mrow> <kb:mo stretchy="false">(</kb:mo> <kb:mn>2.5</kb:mn> <kb:mi>–</kb:mi> <kb:mn>17.5</kb:mn> <kb:mo stretchy="false">)</kb:mo> <kb:mo>×</kb:mo> <kb:msup> <kb:mrow> <kb:mn>10</kb:mn> </kb:mrow> <kb:mrow> <kb:mo>−</kb:mo> <kb:mn>3</kb:mn> </kb:mrow> </kb:msup> </kb:mrow> </kb:math> depending on the <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline"> <ob:msup> <ob:mi>γ</ob:mi> <ob:mo>′</ob:mo> </ob:msup> </ob:math> mass.